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refactor(transformer): exponentiation transform: split into 2 paths (#…
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…6316)

Exponentiation transform has 2 paths for when left-hand side of `**=` is an identifier, and when it's a member expression. But these two paths are mixed up together in the `convert_assignment_expression`, `explode`, and `get_obj_ref` functions, with each branching on the same condition (is it an identifier or a member expression?).

Refactor to separate out these 2 code paths, and make the logic easier to follow.
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@overlookmotel
overlookmotel committed Oct 6, 2024
1 parent eb1d0b8 commit 7d93b25
Showing 1 changed file with 159 additions and 132 deletions.
291 changes: 159 additions & 132 deletions crates/oxc_transformer/src/es2016/exponentiation_operator.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -58,7 +58,7 @@ impl<'a, 'ctx> ExponentiationOperator<'a, 'ctx> {
}

impl<'a, 'ctx> Traverse<'a> for ExponentiationOperator<'a, 'ctx> {
// NOTE: Bail bigint arguments to `Math.pow`, which are runtime errors.
// Note: Do not transform to `Math.pow` with BigInt arguments - that's a runtime error
fn enter_expression(&mut self, expr: &mut Expression<'a>, ctx: &mut TraverseCtx<'a>) {
match expr {
// `left ** right`
Expand All @@ -80,7 +80,21 @@ impl<'a, 'ctx> Traverse<'a> for ExponentiationOperator<'a, 'ctx> {
return;
}

self.convert_assignment_expression(expr, ctx);
match &assign_expr.left {
AssignmentTarget::AssignmentTargetIdentifier(_) => {
self.convert_assignment_to_identifier(expr, ctx);
}
// Note: We do not match `AssignmentTarget::PrivateFieldExpression` here.
// From Babel: "We can't generate property ref for private name, please install
// `@babel/plugin-transform-class-properties`".
// TODO: Ensure this plugin interacts correctly with class private properties
// transform, so the property is transformed before this transform.
AssignmentTarget::StaticMemberExpression(_)
| AssignmentTarget::ComputedMemberExpression(_) => {
self.convert_assignment_to_member_expression(expr, ctx);
}
_ => {}
}
}
_ => {}
}
Expand All @@ -98,28 +112,162 @@ impl<'a, 'ctx> ExponentiationOperator<'a, 'ctx> {
*expr = Self::math_pow(binary_expr.left, binary_expr.right, ctx);
}

/// Convert `AssignmentExpression`.
// `left **= right` -> `left = Math.pow(left, right)`
fn convert_assignment_expression(
/// Convert `AssignmentExpression` where assignee is an identifier.
///
/// `left **= right` transformed to:
/// * If `left` is a bound symbol:
/// -> `left = Math.pow(left, right)`
/// * If `left` is unbound:
/// -> `var _left; _left = left, left = Math.pow(_left, right);`
///
/// Temporary variable `_left` is to avoid side-effects of getting `left` from running twice.
fn convert_assignment_to_identifier(
&mut self,
expr: &mut Expression<'a>,
ctx: &mut TraverseCtx<'a>,
) {
let Expression::AssignmentExpression(assign_expr) = expr else { unreachable!() };
let assign_target = &mut assign_expr.left;
let AssignmentTarget::AssignmentTargetIdentifier(ident) = assign_target else {
unreachable!()
};

let mut nodes = ctx.ast.vec();
let Some(Exploded { reference, uid }) =
self.explode(&mut assign_expr.left, &mut nodes, ctx)
else {
return;

let symbol_id = ctx.symbols().get_reference(ident.reference_id().unwrap()).symbol_id();
// Make sure side-effects of evaluating `left` only happen once
let uid = if let Some(symbol_id) = symbol_id {
// This variable is declared in scope so evaluating it multiple times can't trigger a getter.
// No need for a temp var.
ctx.ast.expression_from_identifier_reference(ctx.create_bound_reference_id(
SPAN,
ident.name.clone(),
symbol_id,
ReferenceFlags::Write,
))
} else {
// Unbound reference. Could possibly trigger a getter so we need to only evaluate it once.
// Assign to a temp var.
let reference = ctx.ast.expression_from_identifier_reference(
ctx.create_unbound_reference_id(SPAN, ident.name.clone(), ReferenceFlags::Read),
);
self.add_new_reference(reference, &mut nodes, ctx)
};

let reference = ctx.ast.move_assignment_target(assign_target);

*expr = Self::create_replacement(assign_expr, reference, uid, nodes, ctx);
}

/// Convert `AssignmentExpression` where assignee is a member expression.
///
/// `obj.prop **= right`
/// * If `obj` is a bound symbol:
/// -> `obj["prop"] = Math.pow(obj["prop"], right)`
/// * If `obj` is unbound:
/// -> `var _obj; _obj = obj, _obj["prop"] = Math.pow(_obj["prop"], right)`
///
/// `obj[name] **= right`
/// * If `obj` is a bound symbol:
/// -> `var _name; _name = name, obj[_name] = Math.pow(obj[_name], 2)`
/// * If `obj` is unbound:
/// -> `var _obj, _name; _obj = obj, _name = name, _obj[_name] = Math.pow(_obj[_name], 2)`
///
/// Temporary variables are to avoid side-effects of getting `obj` or `name` being run twice.
///
/// TODO(improve-on-babel):
/// 1. If `name` is bound, it doesn't need a temp variable `_name`.
/// 2. `obj.prop` does not need to be transformed to `obj["prop"]`.
/// We currently aim to produce output that exactly matches Babel, but we can improve this in future
/// when we no longer need to match exactly.
fn convert_assignment_to_member_expression(
&mut self,
expr: &mut Expression<'a>,
ctx: &mut TraverseCtx<'a>,
) {
let Expression::AssignmentExpression(assign_expr) = expr else { unreachable!() };

let mut nodes = ctx.ast.vec();
let Exploded { reference, uid } =
self.explode_member_expression(&mut assign_expr.left, &mut nodes, ctx);

*expr = Self::create_replacement(assign_expr, reference, uid, nodes, ctx);
}

fn create_replacement(
assign_expr: &mut AssignmentExpression<'a>,
reference: AssignmentTarget<'a>,
uid: Expression<'a>,
mut nodes: Vec<'a, Expression<'a>>,
ctx: &mut TraverseCtx<'a>,
) -> Expression<'a> {
let right = ctx.ast.move_expression(&mut assign_expr.right);
let right = Self::math_pow(uid, right, ctx);
let assign_expr =
ctx.ast.expression_assignment(SPAN, AssignmentOperator::Assign, reference, right);
nodes.push(assign_expr);

*expr = ctx.ast.expression_sequence(SPAN, nodes);
ctx.ast.expression_sequence(SPAN, nodes)
}

fn explode_member_expression(
&mut self,
node: &mut AssignmentTarget<'a>,
nodes: &mut Vec<'a, Expression<'a>>,
ctx: &mut TraverseCtx<'a>,
) -> Exploded<'a> {
let member_expr = node.to_member_expression_mut();

// Make sure side-effects of evaluating `obj` of `obj.ref` and `obj[ref]` only happen once
let obj = match member_expr {
MemberExpression::ComputedMemberExpression(e) => &mut e.object,
MemberExpression::StaticMemberExpression(e) => &mut e.object,
// This possibility is ruled out in `enter_expression`
MemberExpression::PrivateFieldExpression(_) => unreachable!(),
};
let mut obj = ctx.ast.move_expression(obj);
// If the object reference that we need to save is locally declared, evaluating it multiple times
// will not trigger getters or setters. `super` cannot be directly assigned, so use it directly too.
let needs_temp_var = match &obj {
Expression::Super(_) => false,
Expression::Identifier(ident) => {
!ctx.symbols().has_binding(ident.reference_id().unwrap())
}
_ => true,
};
if needs_temp_var {
obj = self.add_new_reference(obj, nodes, ctx);
}

let computed = member_expr.is_computed();
let prop = self.get_prop_ref(member_expr, nodes, ctx);
let optional = false;
let obj_clone = Self::clone_expression(&obj, ctx);
let (reference, uid) = match &prop {
Expression::Identifier(ident) if !computed => {
let ident = IdentifierName::new(SPAN, ident.name.clone());
(
// TODO:
// Both of these are the same, but it's in order to avoid after cloning without reference_id.
// Related: https://github.com/oxc-project/oxc/issues/4804
ctx.ast.member_expression_static(SPAN, obj_clone, ident.clone(), optional),
ctx.ast.member_expression_static(SPAN, obj, ident, optional),
)
}
_ => {
let prop_clone = Self::clone_expression(&prop, ctx);
(
ctx.ast.member_expression_computed(SPAN, obj_clone, prop_clone, optional),
ctx.ast.member_expression_computed(SPAN, obj, prop, optional),
)
}
};
Exploded {
reference: AssignmentTarget::from(
ctx.ast.simple_assignment_target_member_expression(reference),
),
uid: Expression::from(uid),
}
}

fn clone_expression(expr: &Expression<'a>, ctx: &mut TraverseCtx<'a>) -> Expression<'a> {
Expand Down Expand Up @@ -150,127 +298,6 @@ impl<'a, 'ctx> ExponentiationOperator<'a, 'ctx> {
ctx.ast.expression_call(SPAN, callee, NONE, arguments, false)
}

/// Change `lhs **= 2` to `var temp; temp = lhs, lhs = Math.pow(temp, 2);`.
/// If the lhs is a member expression `obj.ref` or `obj[ref]`, assign them to a temporary variable so side-effects are not computed twice.
/// For `obj.ref`, change it to `var _obj; _obj = obj, _obj["ref"] = Math.pow(_obj["ref"], 2)`.
/// For `obj[ref]`, change it to `var _obj, _ref; _obj = obj, _ref = ref, _obj[_ref] = Math.pow(_obj[_ref], 2);`.
fn explode(
&mut self,
node: &mut AssignmentTarget<'a>,
nodes: &mut Vec<'a, Expression<'a>>,
ctx: &mut TraverseCtx<'a>,
) -> Option<Exploded<'a>> {
let (reference, uid) = match node {
AssignmentTarget::AssignmentTargetIdentifier(_) => {
let obj = self.get_obj_ref(node, nodes, ctx).unwrap();
let ident = ctx.ast.move_assignment_target(node);
(ident, obj)
}
match_member_expression!(AssignmentTarget) => {
let obj = self.get_obj_ref(node, nodes, ctx)?;
let member_expr = node.to_member_expression_mut();
let computed = member_expr.is_computed();
let prop = self.get_prop_ref(member_expr, nodes, ctx);
let optional = false;
let obj_clone = Self::clone_expression(&obj, ctx);
let (reference, uid) = match &prop {
Expression::Identifier(ident) if !computed => {
let ident = IdentifierName::new(SPAN, ident.name.clone());
(
// TODO:
// Both of these are the same, but it's in order to avoid after cloning without reference_id.
// Related: https://github.com/oxc-project/oxc/issues/4804
ctx.ast.member_expression_static(
SPAN,
obj_clone,
ident.clone(),
optional,
),
ctx.ast.member_expression_static(SPAN, obj, ident, optional),
)
}
_ => {
let prop_clone = Self::clone_expression(&prop, ctx);
(
ctx.ast
.member_expression_computed(SPAN, obj_clone, prop_clone, optional),
ctx.ast.member_expression_computed(SPAN, obj, prop, optional),
)
}
};
(
AssignmentTarget::from(
ctx.ast.simple_assignment_target_member_expression(reference),
),
Expression::from(uid),
)
}
_ => return None,
};
Some(Exploded { reference, uid })
}

/// Make sure side-effects of evaluating `obj` of `obj.ref` and `obj[ref]` only happen once.
fn get_obj_ref(
&mut self,
node: &mut AssignmentTarget<'a>,
nodes: &mut Vec<'a, Expression<'a>>,
ctx: &mut TraverseCtx<'a>,
) -> Option<Expression<'a>> {
let reference = match node {
AssignmentTarget::AssignmentTargetIdentifier(ident) => {
let reference = ctx.symbols().get_reference(ident.reference_id().unwrap());
if let Some(symbol_id) = reference.symbol_id() {
// this variable is declared in scope so we can be 100% sure
// that evaluating it multiple times won't trigger a getter
// or something else
return Some(ctx.ast.expression_from_identifier_reference(
ctx.create_bound_reference_id(
SPAN,
ident.name.clone(),
symbol_id,
ReferenceFlags::Write,
),
));
}
// could possibly trigger a getter so we need to only evaluate it once
ctx.ast.expression_from_identifier_reference(ctx.create_unbound_reference_id(
SPAN,
ident.name.clone(),
ReferenceFlags::Read,
))
}
match_member_expression!(AssignmentTarget) => {
let expr = match node.to_member_expression_mut() {
MemberExpression::ComputedMemberExpression(e) => &mut e.object,
MemberExpression::StaticMemberExpression(e) => &mut e.object,
// From Babel: "We can't generate property ref for private name, please install
// `@babel/plugin-transform-class-properties`".
// TODO: Ensure this plugin interacts correctly with class private properties
// transform, so the property is transformed before this transform.
MemberExpression::PrivateFieldExpression(_) => return None,
};
let expr = ctx.ast.move_expression(expr);
// the object reference that we need to save is locally declared
// so as per the previous comment we can be 100% sure evaluating
// it multiple times will be safe
// Super cannot be directly assigned so lets return it also
if matches!(expr, Expression::Super(_))
|| matches!(&expr, Expression::Identifier(ident) if ident
.reference_id
.get()
.is_some_and(|reference_id| ctx.symbols().has_binding(reference_id)))
{
return Some(expr);
}

expr
}
_ => return None,
};
Some(self.add_new_reference(reference, nodes, ctx))
}

/// Make sure side-effects of evaluating `ref` of `obj.ref` and `obj[ref]` only happen once.
fn get_prop_ref(
&mut self,
Expand All @@ -289,7 +316,7 @@ impl<'a, 'ctx> ExponentiationOperator<'a, 'ctx> {
MemberExpression::StaticMemberExpression(expr) => {
ctx.ast.expression_string_literal(SPAN, expr.property.name.clone())
}
// This possibility is ruled out in earlier call to `get_obj_ref`
// This possibility is ruled out in `enter_expression`
MemberExpression::PrivateFieldExpression(_) => unreachable!(),
}
}
Expand Down

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